Skin stem cell hypotheses and long term clone survival--explored using agent-based modelling.

Epithelial renewal in skin is achieved by the constant turnover and differentiation of keratinocytes. Three popular hypotheses have been proposed to explain basal keratinocyte regeneration and epidermal homeostasis: 1) asymmetric division (stem-transit amplifying cell); 2) populational asymmetry (progenitor cell with stochastic fate); and 3) populational asymmetry with stem cells. In this study, we investigated lineage dynamics using these hypotheses with a 3D agent-based model of the epidermis. The model simulated the growth and maintenance of the epidermis over three years. The offspring of each proliferative cell was traced. While all lineages were preserved in asymmetric division, the vast majority were lost when assuming populational asymmetry. The third hypothesis provided the most reliable mechanism for self-renewal by preserving genetic heterogeneity in quiescent stem cells, and also inherent mechanisms for skin ageing and the accumulation of genetic mutation.

Safety assessment of enzyme-containing personal cleansing products: exposure characterization and development of IgE antibody to enzymes after a 6-month use test.

BACKGROUND: Enzyme-containing personal cleansing products were being considered for the consumer market. Although enzymes have been marketed safely for many years as ingredients in laundry products, their use in a personal cleansing application represented a new type of exposure for consumers that was not supported by the historical safety data. An exposure assessment and additional safety data would be needed before marketing to ensure consumer safety. OBJECTIVE: The work in this paper was designed to evaluate the potential for inhalation exposure to the enzyme during use of this new product while showering. Then a clinical trial was conducted to determine whether or not the level, duration, and routes of exposure encountered during use of this product would induce a Type I sensitization response to the enzyme. METHODS: Exposure was assessed during normal showering activities by collecting air samples with both high volume and personal samplers and quantitating enzyme levels with an ELISA. To assess the potential for sensitization, panelists were asked to use a prototype protease-containing bar product for all personal cleansing tasks and to keep a use diary reporting any associated symptoms. Physical and dermatologic examinations and skin prick tests with enzyme were conducted before the test commenced and at 2-month intervals. RESULTS: Exposure assessment results showed that airborne enzyme levels were primarily dependent on the concentration of the enzyme in the personal cleansing product. Mean values for total airborne enzyme protein ranged from 5.7 to 11.8 ng/m3 when enzyme concentration, time of use, and measurement technique remained constant. After 6 months of at-home product use, four of 61 test subjects using the enzyme-containing bar had positive skin prick test responses when tested with the enzyme. The skin prick test data were supplemented with serologic analyses, which detected IgE specific for the protease enzyme. None of these subjects showed any clinical symptoms indicative of allergic reaction. CONCLUSION: The ability of enzymes to induce development of allergic antibodies in this study led to the conclusion that this prototype enzyme-containing personal cleansing bar would represent an inappropriate use of enzymes in a consumer product application. The likelihood of both induction of an immunologic response and subsequent elicitation of allergy symptoms in a small but significant fraction of the user population was high. This finding resulted in the decision to halt further development of this prototype.

Percutaneous penetration of N-nitroso-N-methyldodecylamine through human skin in vitro: application from cosmetic vehicles.

The human skin penetration of N-nitroso-N-methyldodecylamine (NDOMA) from isopropyl myristate (IPM) and two vehicles representative of cosmetic/personal care formulations was determined in vitro. When applied as an infinite dose in IPM (1 microgram/microliter) the average total absorption over 48 hr was 0.10 +/- 0.01% of the applied dose (all data are expressed as means +/- SE). When applied as a finite dose in a representative oil-in-water emulsion formulation the average total absorption over 48 hr was 4.66 +/- 0.76% of the applied dose. When applied as a finite dose in a representative shampoo formulation for 10 min, followed by rinsing (to represent in-use exposure conditions), the average total absorption over 48 hr was 0.75 +/- 0.17% of the applied dose. Approximately 72% of the NDOMA in the applied shampoo formulation was removed by rinsing. The overall data indicated that NDOMA could penetrate the skin but that penetration was low. The rate and extent of absorption, however, could be affected by differences in the vehicle of application, time of exposure and whether the formulation is (and the conditions are designed to mimic) a rinse-off or leave-on product.

Percutaneous penetration of dimethylnitrosamine through human skin in vitro: application from cosmetic vehicles.

Human skin penetration of N-dimethylnitrosamine (DMN) from three vehicles has been determined in vitro. When applied as an infinite dose in isopropyl myristate (IPM, 1 microgram/microliter) the average total absorption over 48 hr was 2.6 +/- 1.2% of the applied dose (all data presented are expressed as means +/- standard errors). When applied as a finite dose in a representative oil-in-water emulsion vehicle the average total absorption over 48 hr was 4.0 +/- 0.3% of the applied dose. When applied as a finite dose in a representative shampoo vehicle for 10 min followed by rinsing (i.e. to represent in-use exposure conditions) the average total absorption over 48 hr was 1.1 +/- 0.1% of the applied dose. Approximately 72% of the DMN in the applied shampoo vehicle was removed by rinsing. There was considerable evaporative loss of DMN from the IPM and oil-in-water emulsion vehicles, such that absorption was complete within 3 hr of application. The overall data indicate that DMN can penetrate the skin rapidly but that in practice the amount actually available for penetration is significantly reduced by high permeant volatility. In contrast, application of N-nitrosodiethanolamine (NDELA) at a concentration of 1 microgram/microliter as an infinite dose generated an average total absorption over 48 hr of 23.6 +/- 6.4%, representing a total flux of 103.9 +/- 28.4 micrograms/cm2. In the case of NDELA, no evaporative loss was evident.

Percutaneous penetration of N-nitrosodiethanolamine through human skin (in vitro): comparison of finite and infinite dose applications from cosmetic vehicles.

N-Nitroso compounds (nitrosamines) have been detected at the parts per billion level in a wide variety of matrices including industrial chemicals, pharmaceuticals, and food. Although N-nitrosodiethanolamine (NDELA) may be detected as an impurity in some cosmetic products, studies on NDELA absorption through human skin have been limited. A study to determine the extent of NDELA absorption following topical application was therefore undertaken to assist in the proper assessment of risk following unintended exposure. NDELA absorption was measured in vitro through human cadaver skin using isopropyl myristate (IPM) and generic prototype personal-care formulations (sunscreen and shampoo) spiked with [14C]NDELA. When applied as a finite dose at a concentration of 0.06% or lower, NDELA absorption was found to be a linear function of concentration. Total absorption at 48 hr ranged from approximately 35 to 65% of the dose and was formulation dependent (IPM > shampoo > or = sunscreen). Absorption occurred relatively rapidly from all formulations and peak rates of absorption were seen within the first 5 hr from the IPM and shampoo formulations. When applied as an infinite dose, total NDELA absorption followed a different rank order (shampoo > or = IPM > sunscreen) and evidence of barrier damage was seen with the shampoo formulation.

Exposure assessment of consumer products: human body weights and total body surface areas to use, and sources of data for specific products.

A thorough understanding of the routes and magnitudes of chemical exposures that consumers experience during the use of a household product is needed as part of a well-founded risk assessment for that product and its components. This review describes some sources of generic consumer data (eg, relevant body weight or total body surface area for a given human age), and exposure-related data (eg, task frequency and duration) for specific product types needed for exposure assessments. The review also contains a discussion of the importance of statistical characterization of the consumer data (eg, does its range follow a normal, log-normal, or other type of distribution?). The importance of examining these data for correlative interactions is emphasized.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment on mechanical function of the rat heart.

Mechanical responses of isolated atria to (-)-isoproterenol and activities of myocardial pyruvate kinase and citrate synthase, enzymes involved in energy metabolism, were assessed in rats 7 days after 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) treatment. Basal tension development by electrically paced left atria was significantly elevated by all doses of TCDD (6.25, 25, or 100 micrograms/kg) when compared to that of vehicle-treated rats with unlimited access to feed. The basal rate of spontaneously beating right atria was significantly depressed in rats receiving 100 micrograms/kg TCDD. In left atria from rats treated with 100 micrograms/kg TCDD, maximal inotropic responses to (-)-isoproterenol and 1-methyl-3-isobutylxanthine were enhanced to the same degree. Right and left atria from rats receiving 100 micrograms/kg TCDD had an increased sensitivity to the chronotropic and inotropic effects of (-)-isoproterenol, respectively. The augmented atrial effects caused by TCDD were not secondary to loss of body weight because pair-fed animals that lost the same amount of weight did not display the responses. The ratio of heart ventricular mass to body weight and the activities of pyruvate kinase and citrate synthase in homogenates of heart ventricular muscle were not affected by TCDD treatment. Thus, overtly toxic doses of TCDD in the rat did not depress mechanical function of the heart.

Effects of perfluorodecanoic acid on hepatic indices of thyroid status in the rat.

Perfluorodecanoic acid (PFDA) alters the circulating level of thyroid hormones, but the physiological significance of this change at the target tissue remains to be defined. To this end, the activities of thyroid-responsive hepatic enzymes were examined in adult male rats 1 week after treatment with a single dose of PFDA (20, 40 or 80 mg/kg). Since PFDA treatment caused a dose-related reduction in feed intake, vehicle-treated rats pair-fed to their counterparts receiving PFDA were used to determine if any of the PFDA-induced alterations in enzyme activity were secondary to hypophagia. Following the administration of PFDA, L-glycerol-3-phosphate dehydrogenase, a liver mitochondrial enzyme sensitive to thyroid status, exhibited a modest increase in activity, whereas that of succinate dehydrogenase, a constitutive mitochondrial marker enzyme, was similar in both PFDA-treated rats and their pair-fed counterparts at all dose levels examined. Activity of cytosolic lactate dehydrogenase was also augmented modestly in livers of rats receiving PFDA. In contrast, activity of cytosolic malic enzyme, a thyroid-responsive enzyme, was increased markedly in PFDA-treated rats. Hepatic activity of glucose-6-phosphate dehydrogenase, which also responds to alterations in thyroid status, exhibited a modest increase with 20 and 40 mg/kg PFDA but was similar in both PFDA-treated rats and their pair-fed counterparts at the 80 mg/kg dose level. Absolute and relative liver mass was elevated in PFDA-treated rats at all dose levels in comparison to the appropriate vehicle-treated pair-fed animals. Total hepatic content of DNA was maintained in PFDA-treated rats at all dose levels, whereas a significant decrease in liver DNA was found in the vehicle-treated rats pair-fed to animals receiving 80 mg/kg PFDA. Following administration of PFDA, protein content per total liver was similar to that of their pair-fed counterparts. Thus, the pattern of activity of thyroid-responsive hepatic enzymes was not compatible with a functional shift toward a lessened thyroid status in rats treated with PFDA.

Hepatic indices of thyroid status in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin.

The functional thyroid status of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats is unknown. Therefore, activities of certain thyroid-responsive enzymes were examined in the livers of adult male Sprague-Dawley rats 1 week after treatment with TCDD (6.25, 25 or 100 micrograms/kg). Activity of the thyroid-responsive flavin L-glycerol-3-phosphate dehydrogenase (per mg mitochondrial protein) was decreased slightly in livers of TCDD-treated rats, while that of succinate dehydrogenase remained unchanged. In contrast, activities (per mg supernatant protein) of three thyroid-responsive NADP-dependent cytosolic enzymes, malic enzyme, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, were increased by TCDD treatment in a dose-dependent manner. Lactate dehydrogenase (activity per mg supernatant protein) was also augmented slightly 1 week after TCDD administration. Liver mass was increased by TCDD treatment in a dose-dependent manner, but DNA content per liver was similar at all doses examined. Total hepatic protein, expressed per liver or mg hepatic DNA, was increased in TCDD-treated rats when compared to their pair-fed counterparts. The decreased activity of the mitochondrial L-glycerol-3-phosphate dehydrogenase, in contrast to the increased activities of the supernatant enzymes, malic enzyme, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, is not consistent with a shift in functional thyroid status following TCDD treatment.

Regulation of hepatic malic enzyme by perfluorodecanoic acid.

Perfluorodecanoic acid (PFDA) administration to adult male rats increased both the activity of hepatic malic enzyme and liver weight in a dose-dependent manner. Hepatomegaly and augmented activity of malic enzyme in liver were apparent within one day following PFDA administration and reached a plateau by three days posttreatment. Malic enzyme quantity per liver in PFDA-treated rats was elevated within one day following dosing and increased continually throughout five days posttreatment. Administration of PFDA to rats in the fed state also led to an increase in the specific activity of hepatic malic enzyme that peaked at three days following dosing. When compared to the fed condition, rats fasted for 48 hours had a decrease in both relative liver weight and the quantity of supernatant protein per liver. The total activity (U/liver) and specific activity of malic enzyme in the liver were also reduced in the fasted state. During the 24 hours after treatment in rats fasted for 48 hours, the body weight as well as the absolute and relative liver weight of animals receiving vehicle declined continuously in the absence of feed. Following the administration of PFDA to fasted rats, body weight was maintained until eight hours posttreatment but then declined at a rate similar to that found with the vehicle-treated group. Absolute and relative liver weight in PFDA-treated rats were increased significantly at eight hours posttreatment when compared to those receiving vehicle, and this increment was maintained throughout the rest of the 24 hours following dosing. While the activity and enzyme content of hepatic malic enzyme decreased in the vehicle-treated group, administration of PFDA to rats fasted for 48 hours prevented their decline. The specific activity of hepatic malic enzyme in 48 hours fasted rats receiving PFDA was also elevated significantly at 16 hours posttreatment. Thus, the administration of PFDA to the adult male rat in both the fed and fasted nutritional states was found to regulate hepatic malic enzyme by not only increasing enzyme quantity but also by augmenting the specific activity, (ie, catalytic state) of the enzyme.

Hypophagia-induced weight loss in mice, rats, and guinea pigs treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin.

C57BL/6 mice treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 360 micrograms/kg) displayed a significant reduction in feed intake and body weight until just before death, when they developed ascites and subcutaneous edema. This caused body weight of the mice that died to suddenly increase during the terminal stage of toxicity. TCDD-treated mice that survived did not develop ascites or edema, and maintained a body weight that was slightly less than that of pair-fed mice. Cumulative lethality in TCDD-treated mice (69%) was greater than that of pair-fed controls (14%). In guinea pigs treated with TCDD (2 micrograms/kg) both the time course and magnitude of hypophagia were closely associated with weight loss. Pair-fed guinea pigs did not lose quite as much weight as TCDD-treated animals because their total body water content was higher. Water intake in pair-fed guinea pigs was greater than that of TCDD-treated animals. The time course and magnitude of lethality tended to be similar in TCDD-treated guinea pigs (81%) and pair-fed controls (64%). In Fischer F-344 rats treated with TCDD (100 micrograms/kg) body weight loss was associated with a reduction in both feed and water intake. The time course and magnitude of weight loss in TCDD-treated and pair-fed rats was essentially identical. Lethality was higher in TCDD-treated rats (95%) than pair-fed control animals (48%). Taken together, these findings suggest that hypophagia is responsible for the loss of adipose and lean tissue in mice, guinea pigs, and rats treated with a LD70-95 dose of TCDD. Under these dosage conditions, weight loss contributes more to the lethality of guinea pigs than to that of Fischer F-344 rats or C57BL/6 mice.